The present invention relates to a dimension measuring apparatus for optically detecting the movement of an optical scale (grid) or, in particular, to a configuration of an optical system and a signal processing system of a dimension measuring apparatus.
In a production line for precision members, demand is strong for in-line non-contact type measurement of the distance traveled by a machining head or the size of a workpiece. As a simple measuring instrument often used for this purpose, an optical scale formed with a multiplicity of fine grids having a binary light transmission distribution of black and white at a predetermined pitch is mounted on a probe, and the moving distance of the probe is optically read.
FIG. 21A shows an example of a configuration of a conventional dimension measuring apparatus 700. The light emitted from a light source 701 such as a white lamp or a light-emitting diode (LED) is radiated on an optical scale 710 through a collimating lens 702. The optical scale 710 is configured with three grids including a moving grid 703 and stationary grids 704A, 704B. The moving grid 703 is mounted on a probe (not shown), and is adapted to move in the directions A, B indicated by arrows with the movement of the probe. The stationary grids 704A, 704B are fixed at a specific position behind the moving grid 703.
The moving grid 703 and the stationary grids 704A, 704B each have a rectangular pattern of binary (black and white) light transmission distribution formed on a glass substrate, and have the same shape and the same pitch. FIG. 21B shows an example 715 of a light transmission distribution of a grid. A black pattern 716 transmitting a small amount of light and a white pattern 718 transmitting a large amount of light are both rectangular in shape and formed alternately. One pitch of the grid has a length of a, and the width of the black pattern and the white pattern are both a/2. Normally, a grid of a=10 xcexcm is used.
The stationary grid 704A is arranged so that it is shifted by one fourth of a pitch of the grid length with respect to the stationary grid 704B, and two photodetectors 705A, 705B are arranged behind the stationary grids 704A, 704B, respectively. The photodetector 705A outputs an A-phase signal 720, and the photodetector 705B outputs a B-phase signal 725. The A-phase signal 720 and the B-phase signal 725 (hereinafter collectively referred to as the two-phase signals) both have the transmitted light intensity changed sinusoidally with the movement of the moving grid 703. Since the grid positions of the stationary grids 704A and 704B are shifted by one fourth of a pitch, the phases of the A-phase signal 720 and the B-phase signal 725 are shifted by xcfx80n/2 from each other.
In the case where the grids having the configuration described above are irradiated with parallel light rays, the distance L between the moving grid 703 and the stationary grids 704A, 704B is required to be set to a specific value (Fourier image distance) determined by the relation between the wavelength of the light source 701 and the length a of one pitch of the grid. The Fourier image is defined as a light transmission distribution substantially equivalent to the geometric shape of the moving grid 703 irradiated with parallel light rays from the light source 701. In the case where the wavelength xcex of the light source is 0.7 xcexcm and the grid pitch length a is 10 xcexcm, L is about 140 xcexcm. The light transmission distribution at a position different from the Fourier image surface has a deteriorated contrast, and therefore the two-phase signals are required to be measured with high contrast by disposing the stationary grids 704A, 704B at the Fourier image distance of the moving grid 703.
The signal processing will be explained with reference to the waveform examples 720 and 725 shown in FIG. 21C. The A-phase signal 720 and the B-phase signal 725 are both a sinusoidal wave which moves one cycle when the moving grid 703 moves one pitch of its grid length. In FIG. 21C, the phase of the A-phase signal 720 is xcfx80/2 ahead of the phase of the B-phase signal 725. An example is the case in which the probe begins to move at a position 741 and stops at a position 742. The moving distance from position 741 to position 742 is the dimension to be measured, and for the measurement, the moving distance equal to an integral multiple of one pitch of the grids and the moving distance not longer than one pitch of the grids are both required to be detected.
A sinusoidal wave number counter 731 in FIG. 21A counts one signal for each cycle of the sinusoidal wave and thereby counts the number (integer) of pitches of the moving grid 703. For example, the number is counted for each period of the sinusoidal wave assuming the mesial intensity position (743, 745, 746, etc.) of the amplitude of the A-phase signal 720 as a trigger point. For detecting the number, it is necessary to determine the direction in which the moving grid 703 moves, and the direction of movement is determined from the lead and lag of the phase between the two-phase signals. In the case where the phase of the A-phase signal 720 is advanced, for example, the number is counted upward, while in the case where the B-phase signal 725 is advanced, the number is counted downward.
The moving distance not more than one pitch of the grid is the distance La between the movement start position 741 and the reference position 743, and the distance Lb between the reference position 746 and the stop position 742. The resolution and accuracy of the dimension measurement are determined by the accuracy of detection of the distances La, Lb. Therefore, it is important how finely one cycle of the sinusoidal wave is segmented for detecting the grid stop position. In view of this, the phase of the sinusoidal wave corresponding to the positions 741 and 742 is detected from the intensity of the sinusoidal wave. For attaining the resolution of 0.1 xcexcm in the case where the length a of one pitch of the grid is 10 xcexcm, for example, the phase is required to be detected by segmenting one pitch of the grid into 100 parts.
A phase quadrant determination unit 732 determines the phase quadrant (1 to 4) of the sinusoidal wave from the relation of the intensity of the two-phase signals between the grid stop positions 741 and 742. It is determined that the phase of the A-phase signal 720 is in the second quadrant (xcfx80n/2 to xcfx80) and the phase of the B-phase signal 725 is in the third quadrant (xcfx80 to 3/2xcfx80) at the position 741. The phase detector 733 standardizes the amplitude of both the A-phase signal 720 and the B-phase signal 725 as the magnitude of xc2x11 and, based on the standardized intensity (Va, Vb), detects the phase of the grid stop positions 741, 742 from the arc tangent (Va/Vb) equation, for example. In the process, a trigonometric function table 734 for storing the tangent values of the trigonometric function is prepared, and the phase is determined by referring to the values in the table.
In the case where the phase detected at the grid stop position 742 is xcfx86, for example, the distance not more than one grid pitch is Lb=axcfx86/(2xcfx80). The same applies to the distance La. In the case where the grid pitch a is segmented into 100 parts for detection, the phase is required to be detected with an error of not more than 3 degrees. The dimension calculation unit 735 calculates the dimension from the sum of the moving distance equal to an integer multiple of one grid pitch detected by the sinusoidal wave signal number counter 731 and the moving distance not more than one grid pitch detected by the phase detector 733. In this way, the conventional dimension measuring apparatus is based on the measurement of the intensity of the two-phase sinusoidal wave signals by generating the same signals.
As described above, the conventional dimension measuring apparatus is so configured that one grid pitch length and the grid shape of both the moving grid and the stationary grid are equalized, and two-phase sinusoidal wave signals of the phases different by xcfx80/2 are generated. For improving the resolution of the dimension measurement, one grid pitch is required to be segmented finely and the stop position of the moving grid is required to be accurately detected. For this purpose, the phase of the sinusoidal wave is detected. In the case where the intensity of the A-phase signal is Va and the intensity of the B-phase signal is Vb, the phase is detected from the arc tangent (Va/Vb). In the area where the intensity of the sinusoidal wave is substantially maximum or minimum, however, the intensity changes broadly (the intensity change is small as compared with the phase change) and therefore the problem is posed that the error of phase calculation increases and the dimension measurement accuracy is reduced.
When calculating the phase from the arc tangent (Va/Vb), the amplitude of the A-phase signal is required to be accurately coincident with that of the B-phase signal. For this purpose, fine adjustment of the gain of the photoelectric conversion and the standardization of the intensity of the two-phase signals are required. Also, when converting the arc tangent value into the phase, it is necessary to refer to a trigonometric function table and calculate a series development equation thereby to calculate the phase. The resulting problem is that both the hardware and the software are complicated. Another problem is that in the case where the interval between the moving grid and the stationary grid undergoes a change and the detected two-phase signals are modulated from the sinusoidal wave, the grating position fails to correspond to the phase, thereby causing an error of dimension measurement.
Further, in the conventional dimension measuring apparatus, the grid pitch is shortened (to 10 xcexcm, for example) to improve the measurement resolution. In the case where one grid pitch length is 10 xcexcm, the measurement with a high contrast of light intensity makes it necessary to maintain the gap between the moving grid and the stationary grid at about 140 xcexcm (Fourier image distance). In such a case, the tolerable clearance for setting the gap is not more than xc2x110 xcexcm. As a result, the problem is also posed that the conditions are severe for setting the grid interval for obtaining a light intensity signal high in contrast. This problem becomes more salient with the decrease in the grid pitch length. Still another problem is that the moving grid is required to be irradiated with parallel light rays using a lens to form a Fourier image, resulting in a bigger optical system.
In view of this, in order to solve the aforementioned various problems caused by the use of two-phase sinusoidal wave signal, an object of the present invention is to detect the position where the grid begins to move or stops with a simple signal processing system based on non-sinusoidal wave signal for an improved measurement accuracy and an improved measurement reliability.
Another object of the invention is to reduce the size of and simplify the optical system by a configuration in which only one-phase signal is generated using only one moving grid and one photodetector.
Still another object of the invention is to reduce the size of and simplify the optical system by a configuration in which only one-phase signal is generated using two-phase gratings including one moving grid, one stationary grid and only one photodetector.
Yet another object of the invention is to detect the moving distance of not more than one grid pitch and the direction of movement the grid is detected with a simple signal processing system based only on a one-phase signal for an improved measurement accuracy and an improved measurement reliability.
A further object of the invention is to differentiate the grid pitch length of the moving grid and the stationary grid, generate a non-sinusoidal wave signal so that its intensity changes substantially linearly and it has a effect of reducing the diffractive expansion of the light, and detect the position at which the grating begins to move or stop based on this signal for an improved measurement accuracy and an improved measurement reliability.
In order to solve the problems described above, a dimension measuring apparatus according to this invention comprises a moving member moved for dimension measurement, grid means for moving with the moving member, optical means for radiating the grid means, a light receiving means for receiving the light radiated from the optical means and transmitted through the grid means and generating a non-sinusoidal signal in accordance with the movement of the moving member, and a processing means for measuring the moving distance of the moving member based on the non-sinusoidal wave signal.
Also, a dimension measuring apparatus according to this invention comprises a moving member moved for dimension measurement, grid means for moving with the moving member and having such a pattern that the light transmission distribution continuously changes along the longitudinal direction, optical means for radiating the grid means, a light receiving unit for receiving the light radiated from the optical means and transmitted through the grating means and generating a one-phase signal in accordance with the movement of the moving member, and a processing means for measuring the moving distance of the moving member based on the one-phase signal.
Further, the one-phase signal is preferably a non-sinusoidal wave signal.
Furthermore, preferably, the apparatus comprises one moving grid having a pattern alternating at a predetermined pitch, and the optical means is a laser optical system for radiating a laser beam having a sheet-like spot shape which has a width smaller than the length of the predetermined pitch.
In addition, preferably, the grid means is configured with a first grid having a translucent portion and an opaque portion alternating at a predetermined pitch along the longitudinal direction, and a second grating having a pattern repeated at a predetermined pitch, wherein the first or second grid moves with the moving member while the other is fixed.
What is more, preferably, the grid means is configured with a moving grid moved with the moving member and having a translucent portion and an opaque portion alternating at a first pitch along the longitudinal direction, and a stationary grid having a pattern repeated at a pitch different from the first pitch.
Also, a dimension measuring apparatus according to this invention comprises a moving member moved for dimension measurement, grid means for moving with the moving member, optical means for radiating the grid means, a light receiving means for receiving the light radiated from the optical means and transmitted through the grating means and generating an output signal in accordance with the movement of the moving member, and a processing means for generating a temporally digitized signal by sampling the output signal and measuring the moving distance of the moving member by counting the number of the digitized signals.
Further, the output signal is preferably a non-sinusoidal wave signal.
Furthermore, preferably, the grid means is configured with a first grid having a translucent portion and an opaque portion alternating at a predetermined pitch along the longitudinal direction, and a second grid having a such a pattern repeated at a predetermined pitch that the light transmission distribution continuously changes along the longitudinal direction, wherein the first or second grid moves with the moving member while the other is fixed as a stationary grid, the output signal is a one-phase signal, and the processing means measures the moving distance of the moving grid not more than a predetermined pitch by counting the number of digitized signals.
In addition, preferably, the grid means is configured with a moving grid moved with the moving member and having a translucent portion and an opaque portion alternating at a first pitch along the longitudinal direction, two stationary grids having a translucent portion and an opaque portion alternating at a second pitch different from the first pitch along the longitudinal direction, the light receiving unit includes two photodetectors corresponding to the two stationary grids, respectively, the output signal is a two-phase signal, and the processing means generates two digitized signals corresponding to the two-phase signals, respectively.
Also, a dimension measuring apparatus according to this invention comprises a moving member moved for dimension measurement, a moving grid for moving with the moving member and having a translucent portion and an opaque portion alternating at a first pitch along the longitudinal direction, two stationary grids having a translucent portion and an opaque portion alternating at a second pitch different from the first pitch along the longitudinal direction and arranged one fourth of the second pitch apart from each other, optical means for radiating the moving grid and the stationary grids, two light receiving units for receiving the light radiated from the optical means and transmitted through the stationary grids and generating two-phase output signals in accordance with the movement of the moving member, and a processing means for measuring the moving distance of the moving member based on the two-phase output signals.
According to this invention, the positions where the grid begins to move and stops are detected by a simple signal processing system based on a non-sinusoidal wave signal without using two-phase sinusoidal wave signals so that the accuracy and reliability of measurement can be improved.
Also, according to this invention, only a non-sinusoidal wave one-phase signal is generated using one moving grid and one photodetector or one moving grid, one stationary grid and one photodetector, and therefore the apparatus can be reduced in size and simplified.
Further, the one-phase signal generated from the photodetector is an asymmetrical signal, and therefore the direction in which the grid moves can be easily detected by the one-phase signal alone.
Furthermore, according to this invention, the grid pitch of the moving grid can broadened as compared with the prior art. Therefore, the diffractive expansion due to the grid has a smaller effect, and it becomes possible to output a non-sinusoidal wave signal having a wider area linearly changing with the intensity of the transmitted light, thereby making it possible to detect the grid position easily with a non-sinusoidal wave signal. Further, the fact that the grid pitch of the moving grid can be increased as compared with the prior art permits the grid to be irradiated directly by the light radiated from the light source. Thus a collimator lens or the like is eliminated, and the apparatus can be reduced in size.
In addition, the invention is so configured that digitized signals are generated from the output signal of the photodetector and the number of the digitized signals is counted. Even in the case where the intensity of the output signal is varied or the output signal assumes a nonlinear form, therefore, the grid position can be detected without a reference table or a complicated calculation.
What is more, according to this invention, two-phase digitized signals generated from two-phase output signals can be selectively used. Therefore, the grid position can be easily detected using the linear area of the signal alone.